Borderlands of Science

by Charles Sheffield

  Positronium takes the logical final step of getting rid of the wasted mass of the nucleus completely. It replaces the proton of the hydrogen atom by a positron, an electron with a positive charge. Positronium, like muonium, has been made in the lab, but it too is unstable. It comes in two varieties, depending on spin alignments. Para-positronium decays in a tenth of a nanosecond. Ortho-positronium lasts a thousand times as long, a full ten-millionth of a second.

  As in the case of our transuranic elements, we rely upon future technology to find some way to stabilize them.

  5.3 The production of energy. Burning and the strength of materials do not sound to have much in common, but they are alike in this: they both depend on the relationship of the outer electrons of atoms to each other. We use the word "burning" to mean not only the combination of another element with oxygen, but to describe the combination of any elements by chemical means. We also include the very rapid burning that would normally be called an explosion.

  We would like to know the maximum possible energy that can be obtained by chemical combination of a fixed total amount of any materials. This information will prove particularly important for space travel.

  One way to find out the energy content of typical fuels using standard oxygen burning is to look up their "caloric value." We can also determine, for any particular material, how much weight of oxygen is needed per unit of fuel. Hence, looking ahead to the needs of Chapter 8, we can calculate how fast the total burned material would travel if the heat produced were entirely converted to energy of motion.

  The result does not give a wide range of answers for many of the best known fuels. Pure carbon (coal) gives an associated velocity of 4.3 kilometers a second. Ethyl alcohol leads to the same value. Methane and ethane have almost identical values, at 4.7 kms/sec. The highest value is achieved with hydrogen, at 5.6 kms/sec.

  These values are more than can be achieved in practice, because all the energy produced does not go into kinetic energy. Otherwise, the combustion products would be at room temperature (assuming they started there—liquid hydrogen and liquid oxygen, which together make an excellent rocket fuel, must be stored at several hundred degrees below zero). Also, from thermodynamic arguments, conversion of heat energy to motion energy can never be one hundred percent efficient.

  Why assume that oxygen must be one component in the fuel? Only because oxygen is in plentiful supply in our atmosphere. However, it turns out that oxygen is a good choice for another reason: it combines fiercely with other elements. It is also relatively light (atomic weight, 16). When we examine other combinations of elements, the only one better than a hydrogen-oxygen combination is hydrogen and fluorine. Fluorine, the next element in the atomic table to oxygen, is a halogen, which as we have already noted means it is strongly reactive. When we burn hydrogen and fluorine, and convert all the energy to motion, we find a velocity of 5.64 kms/sec.

  This is a very small gain over hydrogen/oxygen, and there are other disadvantages to using fluorine. The result of the combustion of hydrogen and oxygen is water, as user-friendly a compound as one can find. The combination of hydrogen and fluorine, however, yields hydrofluoric acid, a most unpleasant substance. Among other things, it dissolves flesh. Release it into the atmosphere in a rocket launch, and you will have the Environmental Protection Agency jumping all over you. The disadvantages of the hydrogen-fluorine combination exceed its advantages. Hydrogen and oxygen provide the best fuel in practice.

  Are there other options to improve performance? One possibility is suggested by something that we noted in discussing the strength of materials. Chemical reactions, like chemical bonds, are decided by the interaction of the electrons that surround the nucleus of an atom. However, the weight of an atom is provided almost completely by the nucleus. We must have protons, to make the atom electrically neutral. But if we want to accelerate a material to high speed, the neutrons in the nucleus are just dead weight.

  We would achieve a higher final speed from combustion if we replaced normal oxygen by some lighter form of it. Such an idea is not impossible, because many elements have something known as isotopes. An isotope of an element has the usual number of protons, but a different number of neutrons. For example, hydrogen comes in three isotopic forms: H1 is "normal" hydrogen, the familiar element with one proton and one electron; H2 has one proton, one neutron, and one electron. It is a stable form, with its own name, deuterium. Finally, H3 has one proton, two neutrons, and one electron. It is slightly unstable, decaying radioactively over a period of years.

  However, this takes us in the wrong direction. We are interested in isotopes with fewer neutrons than usual, not more. Oxygen has a total of eight isotopes. The most common form of the atom, O16, has eight protons, eight neutrons, and eight electrons. The four heavier isotopes, O17, O18, O19, and O20, all have more neutrons and are of no interest. There is, however, an isotope O13, with only five neutrons. If we use this in place of normal oxygen, the maximum speed associated with hydrogen-oxygen combustion increases from 5.6 kms/sec to 6.15 kms/sec. Unfortunately, O13 decays radioactively in a fraction of a second; however, O14 is longer-lived, and its use gives a maximum speed of 5.95 kms/sec. The similar use of a lighter isotope of fluorine, F17, gives a speed of 5.8 kms/sec.

  It seems fair to say that 6 kms/sec provides an absolute upper limit for an exhaust speed generated using chemical fuels. Putting on our science fiction hats, can we see any possible way to do better than this?

  Chemical combustion involves two atoms, originally independent, that combine to share one or more of their electrons. Also, as we have seen, neutrons take no part in this process. They just provide useless weight. We would therefore expect the ideal chemical fuel would be one in which no neutrons are involved, and in which the energy contribution from the electrons is as large as possible.

  The best conceivable situation should thus involve only hydrogen (H1, a single proton with no neutron), and obtain the largest possible energy release involving an electron. This occurs when a free electron approaches a single proton, to form a neutral hydrogen atom. The energy release for this case is well-known. It is termed the ionization potential for hydrogen, and it is measured in a particular form of unit known as an electron volt. One electron volt (shortened to eV) is the energy required to move an electron a distance of one centimeter in an electric field of one volt. That sounds like a very strange choice of unit, but it proves highly convenient in the atomic and nuclear world, where most of the numbers we have to deal with are nicely expressed in electron volts. The masses of nuclear particles, recognizing the equivalence of mass and energy, are normally written in eV or MeV (million electron volts) rather than in kilograms or some other inconveniently large unit (an electron masses only 9.109310-31 kilograms).

  The ionization potential of hydrogen is 13.6 eV. The mass of an electron is 0.511 MeV, and of a proton 938.26 MeV. Knowing these facts and nothing else, we have enough to calculate the maximum speed obtained when neutral hydrogen forms from a proton and a free electron. Write the kinetic energy of the product as 1/2mv2, where m is the mass of electron plus proton, and so equals 938.77 MeV. To convert this from the form of an energy to a mass, we invoke E=mc2 from Chapter 2, and divide by c2. The energy provided by the electron is 13.6 eV. Equating these two, we have 1/2x938.7731,000,000 (v/c)2=13.6. Using c=300,000 kms/sec and solving for v, v=51.06 kms/sec.

  This is the absolute, ultimate maximum velocity we can ever hope to achieve using chemical means. It is also surely unattainable. To do better, or even as well in practice, we must turn to the realm of physics and the violent processes of the subatomic world.

  Orders of magnitude more energy are available there. To give an example: the ionization potential of hydrogen is 13.6 eV, so this is the energy released when a free electron and a free proton combine to form a hydrogen atom. The nuclear equivalent, combining two protons and two neutrons to form a helium nucleus, yields 28 MeV—over two million times as much.

  5.4 Organic and inorgani
c: building an alien. Those grandparents of modern chemistry, the alchemists of five hundred years ago, had a number of things on their wish list. One, however, dominated all the others. The alchemists sought the philosopher's stone, able to convert base metals to gold.

  Claiming to be able to transmute metals, and failing, had stiff penalties in the fifteenth and sixteenth centuries. Marco Bragadino was hanged by the Elector of Bavaria, William de Krohnemann by the Margrave of Bayreuth, David Benther killed himself before he could be executed by Elector Augustus of Saxony, and Marie Ziglerin, one of the few female alchemists, was burned at the stake by Duke Julius of Brunswick. Frederick of Wurzburg had a special gallows, gold painted, for the execution of those who promised to make gold and failed.

  The inscription on a gibbet where an alchemist was hanged read: "Once I knew how to fix mercury, and now I am fixed myself."

  Today, we know that the philosopher's stone is a problem not of chemistry, but of physics. The transmutation of elements was first shown to be possible in the early 1900s, by Lord Rutherford, when he demonstrated how one element could change to another by radioactive decay, or through bombardment with subatomic particles. For this achievement, the physicist Rutherford—ironically, and to his disgust—was awarded the 1908 Nobel Prize. In chemistry.

  Next on the alchemists' list was the universal solvent, capable of dissolving any material. That problem has vanished with the advance of chemical understanding and knowledge of the structure of compounds. Today, we have solvents for any given material. Aqua regia, known to the alchemists, is a mixture of one part concentrated nitric acid with three parts concentrated hydrochloric acid. It will dissolve most things, including gold. Hydrofluoric acid is only a moderately strong acid (unlike, say, hydrochloric acid) but it will dissolve glass. However, even today we have no single, universal solvent.

  The old question when considering the problem still exists: If you did make the solvent, what would you keep it in?

  The third item on the alchemist's list of desirable discoveries was the elixir of life. This would, when drunk or perhaps bathed in, confer perpetual youth. The quest for it not only occupied the alchemists in their smoky laboratories, but sent explorers wandering the globe. Juan Ponce de Leon was told by Indians in Puerto Rico that he would find the Fountain of Youth in America. He sailed west and discovered not the fountain but Florida, a region today noted less for perpetual youth than for perpetual old age. Cosmetic surgery, aerobics, and vitamins notwithstanding, the elixir eludes us still. But as we will see in the next chapter, we may be on the threshold of a breakthrough.

  The three alchemical searches are often grouped together, but the third one is fundamentally different from the other two. The first pair belong totally to the chemical world. The elixir of life crosses the borderline, to the place where chemistry interacts with an organism—humans, in this case—to produce a desired effect.

  Five hundred years ago, people were certainly doing this in other ways. That is what medical drugs are all about. However, there was a strong conviction that living organisms were not just an assembly of chemicals. Plants and animals were thought to be basically different from inorganic forms. They contained a "vital force" unique to living things.

  It was easy to hold this view when almost every substance found in the human body could not be made in the alchemist's retorts. The doubts began to grow when chemists such as the Frenchman Chevreul were unable to detect any differences between certain fats occurring in both plants and animals. The key step was taken in 1828, when Friedrich Wöhler was able to synthesize urea, a substance never before found outside a living organism (actually, this is not quite true; urea had been prepared in 1811 by John Davy, but not recognized).

  From that beginning, the chemists of the nineteenth and twentieth centuries one by one produced, from raw materials having nothing to do with plants or animals, many of the sugars, proteins, fats, and vitamins found in the bodies of animals and humans. With their success, it slowly became clear why vitalism had seemed reasonable for so long. The molecules of simple compounds are made up of a few atoms; carbon dioxide, for example, is one atom of carbon and two of oxygen. Copper sulfate is one atom of copper, one of sulfur, and four of oxygen. By contrast, many of the molecules of our bodies contain thousands of atoms. The difference between the molecules of living things and those of nonliving materials is largely one of scale.

  More than that, biological compounds depend for their properties very much on the way they are constructed. Two big molecules can have exactly the same number of atoms of each element, but because of their different connecting structure they have totally different properties (such molecules, like in composition yet unalike in structure, are called isomers). Wöhler's success with urea was due at least in part to the fact that it is, as biological molecules go, simple and small, containing only eight atoms. In fact, urea is not so much a building block of a living organism, as a convenient way of dealing with the excretion of ammonia, an undesirable by-product of other reactions.

  Chemists noticed one other thing. The big molecules of biochemistry all seem to contain carbon. In fact, the presence of carbon is so strong an indicator of organic matter, the terms "organic" and "inorganic" in chemistry have nothing to do with the origin of a material. Organic chemistry is, quite simply, the chemistry of materials that contain carbon. Inorganic chemistry is everything else. The distinction is not quite foolproof. Few people would refer to the study of a very simple molecule, such as carbon dioxide or methane (CH4), as organic chemistry. They reserve the term for the study of substantial molecules that contain carbon. "Biogenic" is a better term than "organic" to describe the chemistry of living things, but today the latter is used to refer to both biogenic and carbon chemistry.

  Why is carbon so important? What is there about carbon that makes it so different, so able to aid in the construction of giant molecules? This question is particularly important if we want to devise alien chemistries. Is it absolutely necessary that alien life-forms, no matter their star or planet of origin, be based on carbon?

  Let us return to the shell model of the atom. Each shell around the nucleus can hold a specific number of electrons, and chemical reactions involve only those electrons in the outermost shell.

  The innermost shell can hold two electrons. The next will hold another eight, for a total of ten. Atoms with spaces in a filled shell match up such electron "holes" with the extra electrons of other substances outside a filled shell.

  Now note the curious situation of carbon. It has six electrons surrounding its nucleus. Thus, it has four extra electrons beyond the two of the first filled shell. On the other hand, it is four electrons short of filling a second shell; thus it has both four extra electrons, and four "holes" to be filled by other electrons. This "ambivalence" (a chemical joke; literally, two valences or strengths) of carbon makes it capable of elaborate and complex combinations with other elements. It is also, as we will see later in this chapter, capable of making elaborate and surprising combinations with itself.

  Is carbon unique? Again we consider the shell model. The third shell can hold another eight electrons. Thus, an element with four electrons more than needed to fill the second shell, namely, fourteen, will be four electrons short of filling the third shell. Like carbon, it will have four extra electrons, and at the same time four electron holes to be filled.

  Element fourteen is silicon. We have been led to it, by a very natural and simple logic, as a substance with the same capacity as carbon to form complex molecules. It can serve as the basis for a "silico-organic" chemistry, the stuff of aliens.

  There will of course be differences between carbon-based and silicon-based life forms. For example, carbon dioxide is a gas at room temperature. Silicon dioxide is a solid with several different forms (quartz, glass, and flint are the most familiar), and remains solid to high temperatures. These differences are an interesting challenge to the writer. Just don't use the carbon/silicon analogy blindly. An alien who b
reathes in oxygen and excretes silicon dioxide is not impossible, but does deserve some explanation.

  5.5 Building a horse. There is no real difference between the chemistry of life and the chemistry of the inanimate world, other than complexity. "Vitalism" is dead. This simple fact demolishes the idea of a "food pill," found in rather old and rather bad science fiction.

  The food pill is an aspirin-sized object that taken twice a day, with water, supplies all the body's needs. Apart from the sheer unpleasantness of the idea (no more pizza, no more veal cordon bleu, no more ice cream), it won't work. The body runs just like any other engine, burning organic fuel to produce energy and waste products. We have seen that there are definite limits to the energy produced by chemical reactions. A couple of small pills a day is not enough, no matter how efficiently they are used. To get by on a food pill, the human body would first have to go nuclear.

  Chemistry and biochemistry are subject to identical physical laws. If we like, we can regard biochemistry as no more than a branch of all chemistry. Conversely, we can use the chemistry of living organisms to perform the functions of general chemistry.

  To take one example, the marine organism known as a tunicate has a curious ability to concentrate vanadium from sea water. If we want vanadium, it makes sense to use this "biological concentrator" (which also provides its own fuel supply and makes its own copies). In science fiction, it is quite permissible to presume that an organism can be developed to concentrate any material at all—gold, silver, uranium, whatever the story demands. It is also reasonable to assume that the principle employed by the tunicate will eventually be understood, so that we can make a "vanadium concentrator" along the same lines, but without the tunicate.